Normally, a natural-circulation boiling water reactor includes a chimney disposed inside the reactor pressure vessel and above the reactor core (refer to Patent Document 1, for example). This chimney is for promoting natural circulation of the coolant in the inside of the reactor pressure vessel by guiding the coolant delivered from the reactor core side in a state of a steam-water two-phase flow to the upper side of the reactor pressure vessel.
In the past, with respect to the chimney, there is one in which plate materials formed of a stainless steel and the like are assembled into a lattice shape, and are formed integrally by welding the positions where the plate materials cross each other (refer to Patent Document 2, for example). As another chimney, there is also one in which square tubes formed of a stainless steel and the like are disposed in a staggered shape, and are formed integrally with embracing the top and bottom thereof being embraced by lattice-shape support plates (refer to Patent Document 3, for example).